RFR: Update GHA to jdk20 [v6]

Mon Dec 12 16:07:12 UTC 2022

On Mon, 12 Dec 2022 15:51:40 GMT, Maurizio Cimadamore <mcimadamore at openjdk.org> wrote:

>> This PR updates the github actions on the jdk20 branch
>
> Maurizio Cimadamore has updated the pull request incrementally with four additional commits since the last revision:
> 
>  - Merge pull request #2 from JornVernee/MAC
>    
>    Get build working on Mac as well
>  - use JDK 17 for Gradle
>  - use JDK 19 for Gradle
>  - Add mac

Marked as reviewed by jvernee (no project role).

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PR: https://git.openjdk.org/jextract/pull/96